Postgrey is a Postfix policy server implementing greylisting. When a
request for delivery of a mail is received by Postfix via SMTP, the
triplet CLIENT_IP / SENDER / RECIPIENT is built. If it is the first
time that this triplet is seen, or if the triplet was first seen less
than 5 minutes, then the mail gets rejected with a temporary error.
Hopefully spammers or viruses will not try again later, as it is
however required per RFC.

The following features compared with greylist.pl from Postfix 2.1.1 are
new: Safe database, automatic maintenance, whitelists, lookup by
subnet, auto-whitelisting of clients, only Berkeley DB and no large
mysql nor postgresql DB needed.